fix(state-db): fail loud when state.db has no agent table instead of skipping everything

A missing *column* is an older agent schema (skip that index); a missing
*table* means the path is not an agent state.db. Without this the new
skip path reported every index 'skipped' with exit 0 on an unrelated DB.

scripts/ensure_state_db_read_indexes.py

@@ -97,6 +97,10 @@ def ensure_read_indexes(db_path, *, confirmed_drained=False, lock_file=None):
                     # messages.timestamp). Skip that index instead of letting its
                     # CREATE fail and roll back the ones this schema supports.
                     present = {r[1] for r in db.execute(f"PRAGMA table_info({table})")}
+                    if not present:
+                        # No such table: this is not an agent state.db (or a
+                        # mistyped path to another database). Fail loud.
+                        raise RuntimeError(f"state.db has no {table!r} table; refusing to continue")
                     if any(col not in present for col, _ in keys):
                         statuses[name] = "skipped"
                         continue

tests/test_pr7445_readonly_review_followups.py

@@ -272,6 +272,19 @@ def test_maintenance_skips_indexes_an_older_schema_cannot_hold(tmp_path, mainten
     assert "idx_sessions_webui_fingerprint" not in names
 
 
+def test_maintenance_fails_loud_on_a_database_without_agent_tables(tmp_path, maintenance_without_fcntl):
+    """``skipped`` is for a missing column only. A database with no ``messages``
+    table is not an agent state.db, so the tool must raise, not report success."""
+    module = maintenance_without_fcntl
+    path = tmp_path / "state.db"
+    with closing(sqlite3.connect(str(path))) as conn:
+        conn.execute("CREATE TABLE unrelated(x)")
+    with pytest.raises(RuntimeError, match="no 'messages' table"):
+        module.ensure_read_indexes(path, confirmed_drained=True)
+    with closing(sqlite3.connect(str(path))) as conn:
+        assert conn.execute("SELECT count(*) FROM sqlite_master WHERE type='index'").fetchone()[0] == 0
+
+
 @pytest.fixture
 def maintenance_without_fcntl(monkeypatch):
     """Import the maintenance module as a platform without ``fcntl`` sees it."""